Recently, functions of program analysis tools relating to computer systems such as a debugger and an ICE (In-Circuit Emulator, registered trademark) have been improved. Analysis and tampering of programs using these tools have become easier.
Arts for enhancing confidentiality protection have been proposed. In one art for example, a program is split into pieces, the pieces are encrypted, and only a necessary portion of the encrypted pieces is decrypted and executed. This makes collection of plain text programs and analysis of the program difficult. Also, in another art as disclosed in Japanese Patent Application Publication No. 2002-536911, an encryption calculation is split into pieces such that the pieces are executed in parallel to make analysis difficult.